Helmholtz energy function

Hermann Ludwig Ferdinand von Helmholtz Definition of A (for arbeit):

$\left.A\right.=U-TS$

where U is the internal energy, T is the temperature and S is the entropy. (TS) is a conjugate pair. The differential of this function is

$\left.dA\right.=dU-TdS-SdT$

From the Second law of thermodynamics one obtains

$\left.dA\right.=TdS-pdV-TdS-SdT$

thus one arrives at

$\left.dA\right.=-pdV-SdT$.

For A(T,V) one has the following total differential

$dA=\left({\frac {\partial A}{\partial T}}\right)_{V}dT+\left({\frac {\partial A}{\partial V}}\right)_{T}dV$

The following equation provides a link between classical thermodynamics and statistical mechanics:

$\left.A\right.=-k_{B}T\ln Q_{NVT}$

See also the Canonical ensemble.